The invention is applicable to any loaded beam for which a defined component of the shear force to which the beam is subjected is to be established, and it is particularly advantageous for use in aeronautics. It is important to be able to measure the weight of an aircraft with an error of less than 1%. This can be done by using sensors capable of measuring the vertical shear force exerted on an axle or a wheel lever in the aircraft undercarriage, since both axles and wheel levers are substantially horizontal beams having one end connected to the aircraft structure and having the other end connected to one or more wheels.
For measurement purposes, such a beam has a pair of lugs projecting substantially horizontally from one of its sides when in the measurement position, with said lugs being separated by a predetermined distance in the longitudinal direction of the beam. The opposite side of the beam has a second pair of lugs disposed symmetrically to the lugs of the first pair about an axial plane of symmetry. Each lug is an integral portion of the beam: it projects transversely from the beam and has a rectangular right cross-section with top and bottom plane faces. These two faces are parallel on any one lug and they are situated in planes extending parallel to the axis of the beam and perpendicular to the direction of the shear force to be measured. In addition, they each have a central hole passing therethrough for mounting the measurement sensor. Because of the deformation due to the couple about the axis of the beam, it is generally necessary to use two sensors mounted on two pairs of lugs on opposite sides of the beam in order to be able to calculate the shear force, and consequently to be able to determine the weight of the aircraft. A single sensor and a single pair of lugs suffice when the couple about the beam axis is negligible.
Heretofore, each sensor (or each intermediate support between a sensor and a lug) has included a base with a plane face which is pressed against the major portion of the bearing face of a lug (generally the top face), by means of a bolt passing through the hole provided for the purpose.
U.S. Pat. No. 4,269,070 shows one example of this method of sensor mounting, and describes a type of displacement sensor suitable for performing measurements of this type.
The present invention does not require a particular type of sensor to be used, in other words the sensor used may be inductive, capacitive, optical, piezo resistive, etc. . . , however, when sensor fixing bases are connected to lugs by means of a fixing device in accordance with the present invention, useful measurements can only be obtained if the sensor measuring axis is properly positioned, for example as defined in the above-mentioned US patent.
The main aim of the present invention, in comparison with conventional devices, is to simultaneously increase the measured displacement and the accuracy with which this displacement is measured for a given amount of beam deformation.